Scoria coarse "sand" for my Haworthias and others

Question

I don't find horticulture-grade pumice available locally. So, I bought some scoria and sifted it. I got three grain sizes, with two shown here:

The third is whatever passed through the finest sieve. It is the finest, still coarser than regular sand but full of dust. I have kept it until I find what to do with it.

My question - Which to use for the following plants: Haworthia, Gasteria, Aloe, Echeveria and its allied species. Is the finer grain of any use?

My climate - Csa according to the köppen climate classification: hot Mediterranean.

Edit: Climatic data for Tel-Aviv in this link. No rainfall in summer (but very humid), high rainfall in winter.

Do I need to add water-retaining mediums, such as peat or vermiculite? I grow my collection items in unglazed pots. Rooted cuttings from trimming are grown in plastic pots.

Answer 1

You don't mention where you are, but peat and vermiculite are two of the least desirable water retention materials to use, in my opinion. Calcined clay, or oil absorbent, pumice, etc are much preferred. The oil absorbent is found in autoparts stores. Coconut coir is superior to peat in that it is much easier to rewet once it dries and it doesn't break down as quickly. Are there any hydroponic stores near you? They usually have this sort of material.